Geothermal pipe collector

ABSTRACT

A geothermal pipe collector is provided. The geothermal pipe collector is made from a polymer composition including more than 50 wt % polyethylene, 0.1 wt %-35 wt % talc and 0.5 wt %-10 wt % carbon black.

TECHNICAL FIELD

The present application relates to pipe collectors for geothermal heat exchange and polymer compositions for improving the properties of geothermal pipe collectors.

BACKGROUND

Geothermal energy is energy stored as heat in the ground. This energy may originate from the hot core of the earth or may be heat generated by the earth surface being exposed to infrared radiation from the sun. Most geothermal installations today use the second category of geothermal energy, i.e. solar energy stored as heat in e.g. water, ground or bedrock.

In a geothermal energy system using circulating fluid, the heat is extracted from the ground (e.g. water or bedrock) using a geothermal pipe collector. In the geothermal pipe collector, the fluid, known as heat transfer medium or heat transfer liquid is circulated such that fluid heated by the geothermal energy is extracted in one end of the geothermal pipe collector, the cooled fluid is then returned in the other end of the geothermal pipe collector such that a closed system is created.

Examples of geothermal energy systems are ground surface heat systems, sea heat systems and borehole heat systems. In ground surface heat systems, a several hundred meter long geothermal pipe collector is buried in the ground at a frostproof depth. In a sea heat system, a similar pipe collector is placed in the sea water and/or on/in the sea bed.

In a borehole heat system, a geothermal pipe collector having two fluid conduits is placed in the bore hole, such that the fluid can be conveyed into the bore hole in a first fluid conduit and conveyed from the bore hole in a second fluid conduit. The geothermal pipe collectors for use in borehole heat systems could be so called U-pipe collectors. A U-pipe collector comprises a separate, closed pipe which is bent such that it forms a U-shape, such that the direction of the fluid conveyed is altered in the bottom of the borehole. An alternative geothermal pipe collector is the so called double U-pipe collector, which comprises two pipes for conveyance of the heat transfer liquid down into the drilled hole which branches off into two pipes that transports the fluid back from the drilled hole and further to a heat pump.

Yet another type of collector used in borehole heat systems is the so called coaxial collector. In the coaxial collector, an inner pipe is arranged in an outer pipe. The pipes are welded together, such that a single unit is formed, and subsequently installed in a drilled hole. The fluid is conveyed down into the borehole in the outer pipe and thus absorbs heat from the borehole. When the fluid reaches the bottom of the borehole it is conveyed up again through the inner pipe. It is desirable to avoid long surface area contact between heated fluid and the cooled fluid, which is why the outer pipe is provided with a larger cross-sectional area, such that a faster flow is achieved in the inner pipe.

The heat or cooling collected by means of any of the collectors described above is used to vaporize or condensate a cooling agent of a heat pump in the system, and thereby heat or cooling is extracted from the circulating fluid.

Due to its resistance against degradation by the environment of the ground, Polyethylene (PE) is a material widely used for the manufacturing of geothermal pipe collectors. However, PE is generally viewed as a thermal insulator with low thermal conductivity, which is a drawback when the material is used in heat exchange applications. For the purpose of decreasing the thermal resistance in the pipe collector, the wall thickness of the pipe collector may be decreased. However, decreasing the wall thickness affects the mechanical properties of the pipe, which may be a disadvantage for fulfilling the requirements of pipe standards and for the handling of the pipe. In EP 2195586 to M. Ojala et al., it is described how the thermal conductivity of a geothermal pipe collector can be increased by creating a turbulent flow of the fluid inside the pipe collector. However, creating a turbulent flow involves the creation of a groove or recess in the wall of the geothermal pipe collector, which decreases the thickness of the wall of the geothermal pipe collector, again affecting some of the mechanical properties of the pipe.

To be able to reduce the length of the pipe used in ground surface heat systems, and reduce the depth of the borehole in borehole heat systems, it would be advantageous to have a design of a geothermal pipe collector with reduced thermal resistance and maintained mechanical properties.

SUMMARY

A geothermal pipe collector is provided. The geothermal pipe collector is made from a polymer composition comprising: more than 50 wt % Polyethylene (PE), 0.1 wt %-35 wt % talc and 0.5 wt %-10 wt % Carbon black (CB). The addition of CB protects the geothermal pipe collector against the natural environment but reduces some of the mechanical properties of the polymer composition. The addition of talc increases the performance of a thermal conductivity thus increasing the heat exchanging capabilities of the geothermal pipe collector making it possible to have the same heat exchanging capabilities with a shorter pipe collector. The addition of talc also increases the relevant mechanical properties of the pipe collector, which makes it possible to have thinner walls, which further decreases the thermal resistance of the pipe collector and thus increases the heat exchange.

According to one embodiment, the geothermal pipe collector is made from a polymer composition comprising 0.1 wt %-3 wt % talc.

According to one embodiment, the geothermal pipe collector is made from a polymer composition comprising 8 wt %-35 wt % talc. By adding more than 8% talc, the density of the pipe collector is increased such that the pipe collector gets a higher density than water and thus sinks in a borehole or in sea water.

According to one embodiment, the geothermal pipe collector is made from a polymer composition comprising 8 wt %-15 wt % talc. This embodiment has a higher density than water and is still flexible enough to easily be coiled.

According to one embodiment, the geothermal pipe collector is made from a polymer composition comprising 8 wt %-12 wt % talc. In the interval 8 wt %-12 wt % talc, the polymer composition have some mechanical properties being substantially the same as PE without the addition of CB.

According to one embodiment, the geothermal pipe collector is made from a polymer composition comprising 0.5 wt %-5 wt % CB or 1.5 wt %-3 wt % CB. Both compositions provide sufficient protection against the natural environment, but a polymer compositions comprising 3 wt %-5 wt % CB have a higher thermal stability.

In any of the embodiments herein, the talc added to the polymer composition may be talc in which the average aspect ratio is above 1.2. Talc with a high aspect ratio further increases the thermal conductivity of the polymer composition.

According to one embodiment, the inner surface of the geothermal pipe collector comprises recesses or protrusions for increasing the turbulence of a medium flowing in the pipe collector and thus the heat exchange in the pipe collector. The recesses or protrusions may extend helically on the inner surface of the pipe collector, in relation to the length axis of the pipe collector, such that a turbulent flow is created in the direction of the length axis of the pipe collector. In one embodiment, the helically extending recesses or protrusions alter direction at least at some portion along the length axis of the pipe collector, such that the direction of the turbulent flow is altered along the length axis of the pipe collector, which introduces further turbulence of the fluid.

The recesses or protrusions on the inner surface of the pipe collector may extend continuously on the inner surface of the pipe collector, such that the pipe collector can be manufactured by means of continuous extrusion.

Please note that any of the polymer compositions or any combinations of additives mentioned herein could be used with any type of geothermal pipe collector without departing from the basic idea of the invention.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawing, in which:

FIG. 1 shows a geothermal U-pipe collector in a borehole,

FIG. 2 is a table of polymer compositions on which experiments have been conducted,

FIG. 3 is a graph showing the density of a polymer composition as a function of the filler contents,

FIG. 4 is a table showing the tensile strength of different polymer compositions,

FIG. 5 is graph showing tensile strength of polymer compositions as a function of the filler content in the compositions,

FIG. 6 a is a graph showing impact resistance as a function of filler content in polymer compositions, notched and edgewise,

FIG. 6 b is a graph showing impact resistance as a function of filler content in polymer compositions, un-notched and flatwise,

FIG. 7 is a graph showing elongation and impact resistance as a function of the filler content in the polymer composition,

FIG. 8 is a graph of thermal conductivity and thermal diffusivity of polymer compositions as a function of the filler content in the polymer composition,

FIG. 9 is a graph of the volumetric heat capacity and specific heat of polymer compositions as a function of the filler content in the polymer composition,

FIG. 10 a is a graph showing the thermal resistance of a pipe collector as a function of the thermal conductivity of the polymer composition of which the pipe is manufactured,

FIG. 10 b is a graph showing the thermal resistance of a pipe collector as a function of the thermal conductivity of the polymer composition of which the pipe is manufactured, for different pipe dimensions,

FIG. 11 is a table showing the weight loss of different polymer compositions,

FIG. 12 is a graph showing the weight of a polymer composition as a function of temperatures to which the polymer composition is exposed,

FIG. 13 is a graph of the particle size of a polymer composition by showing the percent of the volume which is made up of particles having a specific diameter,

FIG. 14 a shows an embodiment of a geothermal pipe collector in which the pipe collector comprise helically extending recesses,

FIG. 14 a shows an embodiment of a geothermal pipe collector, in which the pipe collector comprise helically extending recesses which alter direction at least at some portion along the length axis of the pipe collector.

DETAILED DESCRIPTION

In the following, a detailed description of embodiments of the invention will be given with reference to the accompanying drawings. It will be appreciated that the drawings are for illustration only and are not in any way restricting the scope of the invention. Thus, any references to directions, such as “up” or “down”, are only referring to the directions shown in the figures.

FIG. 1 shows the principle for a conventional U-pipe collector. According to this principle, a continuous, sealed Polyethylene (PE) pipe 1 is arranged in a drilled borehole 2. The pipe 1 is preferably made as a single continuous pipe extruded in a plastic extruder. The pipe 1 forms a U-shaped bend 3 in the end towards the bottom 4 of the borehole, such that the fluid conveyed into the borehole 2 is conveyed up again after reaching the bottom of the borehole 2. This particular system is known as a “U-pipe collector” as the bend forms a U-shape. FIG. 1 only shows the principle, the U-shape of the pipe collector may be a separate part to which a first and second portion 1′, 1″ of the pipe is welded. The upper part 5 of the collector system is usually terminated in a manhole at ground level 6, from where the collector pipes 1, 1′, 1″ are connected to a heat pump (not shown). During assembly, the pipe is fed into the borehole 2 manually or by means of a pipe feeder. The pipe feeder typically comprises pulleys feeding the pipe, and the feeder thus bends the pipe while pressing it into the borehole. The feeding thus places substantial strain on the pipe and the pipe thus needs to be both flexible and capable of handling considerable strain.

To withstand the ground environment and the strain induced by the feeder, the geothermal pipe collector is made from a polymer composition based on PE. A polymer composition is to be understood as any compounded material comprising at least one polymer material in some quantity, and a filler is to be understood as a material in the polymer composition other than the main polymer (herein PE). The fillers described herein are talc and CB, and talc is the filler in instances in the graphs where the filler is not specified.

For the purpose of increasing the outdoor stability and in particular the UV resistance of the PE, Carbon black (CB) is added to the polymer composition. CB is a form of amorphous carbon produced by incomplete combustion of petroleum products. CB is an economic additive which effectively increases the UV resistance of PE even at low concentrations (such as between 0.5% and 3%). CB can also be used to increase the thermal stability of PE, which is important when the pipe collectors are used in high temperature applications. The drawback however, is that CB reduces some of the mechanical properties of PE, making the material hard and brittle.

Talc is a mineral composed of hydrated magnesium silicate arranged in three disc shaped layers. In the middle, there is a layer of magnesium-oxygen/hydroxyl octahedra, while the two outer layers are composed of silicon-oxygen tetrahedra. These layers are kept together only by van der Waals' forces, and the layers have the ability to slip over each other easily, which makes talc the softest known mineral, measured as 1 on the Mohs hardness scale. The talc's unique characteristics such as softness, chemical inertness, slipping, oil and grease absorption, whiteness, availability, and its rather low price, makes it a promising material to be used as a filler.

For the purpose of decreasing the thermal resistance of the geothermal pipe collector and enhancing the mechanical properties of the polymer composition, talc is added to the polymer composition. The talc filled polymer composition decreases the thermal resistance of the geothermal pipe collector by increasing the thermal conductivity of the material and enabling the reduction of the wall thickness of the geothermal pipe collector, thereby increasing the heat exchange.

In one embodiment, the geothermal pipe collector is made from a PE based polymer composition comprising: 0.1 wt %-35 wt % talc and 0.5 wt %-10 wt % CB (and the remaining part PE). The CB protects the geothermal pipe collector against the elements of nature, and to have a sufficient protection against UV-radiation, at least 0.5% should be included in the composition. The addition of CB also improves the heat stability of the polymer composition. However, as previously mentioned, the CB negatively affects some of the mechanical properties of PE, in particular the impact resistance. The added talc increases the thermal conductivity of the polymer composition as well as the modulus of elasticity and the tensile strength, and increases the density of the polymer composition. Apart from that, the addition of talc reduces equipment wear during processing, decreases shrinkage, and improves the product machinability. Also, the addition of talc reduces the specific heat capacity of the composition, which makes it possible to increase production speed.

In some applications, additional additives may be needed in the polymer compositions for further increasing the thermal stability or mechanical properties of the PE. Such additives may include Kaolin clay, silica and calcium carbonate, or dye for obtaining a pipe collector of a specific color.

In one embodiment the geothermal pipe collector is made from a polymer composition comprising 0.1 wt %-3 wt % talc. The conductivity of the polymer composition may be increased at small levels of talc, as the crystallinity of the polymer composition is increased.

In one embodiment, the geothermal pipe collector is made from a polymer composition comprising 8 wt %-35 wt % talc. Above 8 wt % talc, the polymer composition has substantially the same impact resistance as PE without the addition of CB (denoted as PEn in the tables and diagrams herein). Above 8% of talc, the polymer composition will have a density of more than 1 (i.e. higher than water), which makes the geothermal pipe collector sink to the bottom of the borehole, which is a clear advantage as no active feeding of the pipe collector is needed. Also, the pipe collector having a density of above 1 could be used in sea heat applications without the need to anchor the pipe collector to the sea bed.

Adding more than 35 wt % of talc in the polymer composition will create a polymer composition having an elasticity modulus too high for normal extruders, and even if the material could be extruded with high pressure, the finished pipe collector would be difficult to coil or feed into the borehole without the risk of breakage, which will make the material very hard to transport and handle. Also, the risk that the material is not properly compounded increases as the amount of talc is increased, creating a risk that large areas of poorly mixed polymer composition will be created, which increases the risk of breakage.

According to one embodiment the polymer composition comprises 8 wt %-15 wt % talc. In this interval, the polymer composition (with 0.5 wt %-3 wt % CB) has density above 1 and substantially the same impact resistance as PE without CB. As can be seen in for example the graph of FIG. 7, the impact resistance of the polymer composition is reduced considerably above 15 wt % talc, which makes the polymer composition less suitable for some geothermal pipe collector applications. In applications benefiting from the wall of the geothermal pipe collector being thin, the polymer composition could comprise 8 wt %-12 wt % talc, which is the interval (depending on the amount of CB in the composition) in which the impact strength is highest at the same time as the density is above 1, making the geothermal pipe collector sink in the borehole or when placed in the sea. When the polymer composition comprises approximately 2.5 wt % CB, the impact resistance, as measured by the Charpy impact test, is reduced by almost 30% (as can be seen in the graph of FIG. 6). For regaining the impact strength of PE without the CB (PEn), between 8 wt %-12 wt % of talc needs to be added to the composition, which, as stated above, also increases the density of the material such that the material will have a density higher than water.

For the purpose of optimizing the talc's effect on the conductivity of the polymer composition, the talc could have a high aspect ratio. The plate-like shape of high aspect ratio talc adds to the enhancement of the impact strength and the thermo-physical characteristics However, when the talc is compounded into the polymer composition, the particles are fractured or altered in shape, which decreases the aspect ratio of the talc particles in the polymer composition. In one embodiments, the polymer composition including high aspect talc means that the aspect ratio of the talc particles on average is above 1.2. In other embodiments, the polymer composition including high aspect talc could mean that the aspect ratio of the talc particles on average is above 1.5, and in yet another embodiments, the polymer composition including high aspect talc could mean that the aspect ratio of the talc particles on average is above 2.

As talc has a plate-like shape with a high aspect ratio and the layers of talc can easily slip over each other during the processing, the polymer can easily fill the spaces between the particles this would happen if there is sufficient shear stress during the pipe processing or compounding. Particulates can therefore be oriented in the flow direction parallel to the axis of the pipe surely particle orientation depends on the nature of the flow field, but it is quite clear from SEM images that particles are well dispersed and oriented with the injection moulding direction. This unique organization allows forming kind of heat channels as result heat can be transferred through, although the particulate did touch completely together to make the true heat channel.

The addition of CB in the polymer composition increases UV resistance of the polymer and increases the thermal stability. However, when above 5 wt % CB has been added to the polymer composition the mechanical properties have decreased substantially, which makes the material very brittle and increases the risk that the material will break during handling, and makes the material hard to transport as a coil. The polymer composition could therefore, according to one embodiment, comprise 0.5 wt %-5 wt % CB (up to 5 wt % for increased thermal stability), and for the purpose of providing sufficient UV-protection comprise 1.5 wt %-3 wt % CB.

Depending on the amount of additives in the polymer composition, in particular CB and talc, the amount of PE in the finalized composition varies. However, the polymer composition preferably comprises more than 75 wt % PE.

Although a single pipe collector in the shape of a U-pipe collector is described with reference to the figures, the polymer compositions described could just as well be used in other types of collectors, such as coaxial collectors or double U-pipe collectors. Any of the geothermal pipe collectors may be adapted for borehole heat systems, ground surface heat systems or sea heat systems or any geothermal cooling system.

Experiments Performed on Polymer Compositions

In the following, tests performed on material compositions will be presented. The specific compositions tested are to be seen as exemplifying embodiments supporting the inventive benefits of the compositions presented herein, and are not be seen as restricting the scope of the present invention.

In the following experimental embodiments, High-Density Polyethylene (HDPE) was used as a matrix material, which was supplied by Unipetrol RPA, Czech-Republic. The HDPE has a melt flow rate (MFI) of 0.4 g/10 min, a Vicat softening temperature of 118° C., and a density of 952 kg/m314. The HDPE contains 2.5 wt % of Carbon black (CB), which was fully precompounded by the supplier. For simplicity, from here onward the material is referred to as “PEc” (HDPE with CB). In order to investigate the effect of this amount of CB, a sample of the same neat HDPE resin without CB was also obtained from the same supplier, and this material is referred to as “PEn” (HDPE neat). The neat HDPE has an MFI of 0.4 g/10 min, a Vicat softening temperature of 122° C., and a density of 942 kg/m321. The PEn was used as a reference to investigate the effect of the CB added on the neat HDPE, and the PEc was used as a reference to study the effect of talc on the HDPE/CB/talc composites.

Commercial talc HAR T84 from Luzenac, France, was used as filler.

FIG. 2 is a table showing the different compositions and indicating the amount of CB and talc in the respective compositions. The compositions shown in the table were prepared by compounding the PEc and the talc at different ratios, in a twin-screw extruder with two side-feeders (ZSK 25 WLE; Cooperion Werner & Pfleiderer, Germany). The temperature profile used was 180-220° C. from feed to die (above the melting temperature of HDPE and well below its decomposition temperature). The PEc was fed into the main hopper with a screw speed of 230 rpm while the talc was fed into the side-feeder with a screw speed of 18 300 rpm. Two individually controlled gravimetric K-tron feeders were used to control the feeding rate, both for the resin and the filler. The throughput was set to 13.67 kg/h for the main feeder, while the throughput for the side-feeder was set to obtain the desired sample composition. The extruded strand was cooled in a water bath and pelletized. The granules were then oven-dried followed by injection molding in an Engel ES 200/110HL Victory with a screw diameter of 30 mm into tensile-testing bars according to ISO standard 572-2/1A. These test bars were also used for the Charpy impact resistance measurement, thermal conductivity measurements, and water absorption testing. For these tests, the samples were cut to the dimensions according to the test standards.

FIG. 3 is a diagram showing specific density as a function of the weight fraction of talc in the polymer composition, clearly showing that the specific density increases when the amount of talc in the polymer composition increases. The increment of density is linearly proportional to the talc content. Point A denotes the polymer composition having a density of 1, which is the composition having approximately 8 wt % of talc (at 2.5 wt % of CB). At levels of talc above 8 wt %, the polymer composition is thus heavier than water and thus sinks in a sea, lake of borehole.

FIG. 4 shows a table describing the influence of the talc loadings on the tensile strength, elongation at yield, and elongation at break, as well as the E-modulus. It can be seen that the tensile strength at yield increased gradually with increasing filler content. In contrast, PEc, which had 2.5% CB, showed a slight decrease in tensile strength compared to HDPE with no filler (PEn). The increase in tensile strength on incorporation of talc was more evident at higher concentrations. The stiffness was increased with both fillers: in the presence of CB, the E-modulus showed a slight increment—about 5% compared to HDPE without filler—whereas the role of talc was more prominent. Despite the fact that the two additives CB and talc are totally different in terms of shape and size, the tensile modulus increased with higher content of either type of particle. To evaluate the significance of differences observed between different composite formulations, the data (for the six talc concentrations from 5% to 35% and PEc as control were analyzed by one-way ANOVA at the 95% confidence level. AP-value for the tensile strength at yield was less than α=0.05, we concluded that the effect was statistically significant.

FIG. 5 shows a graph of the tensile strength as a function of the filler content. It can be noted that the tensile strength at yield increased gradually with increasing filler content. In contrast, PEc, which had 2.5 wt % CB, showed a slight decrease in tensile strength compared to HDPE with no filler (PEn). The increase in tensile strength on incorporation of talc was more evident at higher concentrations. To evaluate the significance of differences observed between different composite formulations, the data (for the six talc concentrations from 5 wt % to 35 wt % and PEc as control) were analyzed by one-way ANOVA at the 95% confidence level. The P-value for the tensile strength at yield was less than α=0.05, we concluded that the effect was statistically significant. An equivalent result, i.e. an increase in tensile strength with an increased filler concentration, was obtained for the tensile strength at break, also shown in FIG. 5. The P-value of <0.05 for the tensile strength at break indicated that there were significant differences between the various composites. Regarding the elongation at yield, as anticipated, the PEn (pure HDPE) had the highest value of all compounds. Generally, a decrease in elongation with an increase in filler content can always be expected due to the fact that the filler added causes a reduction in chain mobility, giving rise to a rapidly decreasing elongation at break. However all the compounds (except PEc with 8% talc) showed a decrease in strain at yield. The stiffness was increased with both fillers: in the presence of CB, the E-modulus showed a slight increment—about 5 % compared to HDPE without filler—whereas the role of talc was more prominent. Despite the fact that the two additives CB and talc are totally different in terms of shape and size, the tensile modulus increased with higher content of either type of particle. This shows that improvement in the stiffness is more due to the fact that rigid particulates restrict the mobility of the chain segments of the macromolecule. Thus, improvement in the stiffness of composites is only weakly dependent on particle size and shape. To a certain degree, the stiffness of the composites relies on the uniform dispersion of the particle in the matrix. It is usually expected that agglomerates are formed when the particle amount is increased, leading to decrease in modulus. But in the present case, the modulus increased even at the highest talc concentration. This indicates that the talc particles are distributed uniformly but not randomly, and not as aggregates, even at the highest concentration.

FIGS. 6 a and 6 b shows impact resistance measured by a Charpy impact test, as a function of filler content in a notched specimen edgewise (FIG. 6 a), and un-notched flatwise (FIG. 6 b). Since the talc particle is platy like, the compound cannot be considered as isotropic material, which is why it is relevant to investigate the impact resistance in two different direction (flatwise and edgewise). The notched specimen includes a small crack such that the specimen shall fail during testing. The Charpy impact test, is a test which determines how much energy a material absorbs during fracture. This absorbed energy provides a measure of a given material's notch toughness. At first, with incorporation of 2.5 wt % of CB in the HDPE, the toughness was steeply reduced by 34%. Then, in the presence of talc, the toughness gradually improved until at 8 wt % talc loading, were the highest value for impact was reached, which was very close to the value for pure HDPE (83 kJ/m218). After that, the impact resistance dropped gradually with an increase in filler loading. Since some materials and composites are more sensitive to notches than others, it is advisable to compare the results for notched and un-notched specimens. The Charpy impact test was also done with un-notched, flat wise direction and the results are shown in FIG. 6 b. In the recent case also, the same trend can be noted that PEc showed a moderate drop in impact resistance compared to PEn (as for the notched, edgewise case). With increase in talc loading, the impact strength increased. Statistical analysis using one-way ANOVA also gave a significant effectiveness of talc on the impact strength compared to the PEc for both impact directions. Further the ANOVA analysis according to Hsu's MCB method (multiple comparisons with the best) showed that the composite with 8 wt % talc was the best of all with 95% confidence interval.

FIG. 7 shows a graph of the impact resistance (Charpy test) and tensile elongation as a function of filler content. As can be seen, the tensile elongation and impact resistance correlate. At first, with incorporation of 2.5 wt % of CB in the HDPE, the toughness was steeply reduced by 34%. Then, in the presence of talc, the toughness gradually improved until at 8 wt % loading the highest value for impact resistance was reached, which was very close to the value for pure HDPE (83 kJ/m218). After that, the impact resistance dropped gradually with an increase in filler loading.

FIG. 8 shows thermal conductivity and thermal diffusivity (thermal conductivity divided by density and specific heat capacity at constant pressure) as a function of filler content. It was found that the thermal conductivity and the thermal diffusivity increased gradually. The enhancement of the thermal conductivity indicates that a percolated particle network was not formed, as we could not achieve the thermal conductivity values of pure talc. At higher filler concentrations, one would expect that the fillers would form thermally conductive percolated networks (instead of isolated thermally conductive particles surrounded by the matrix), and heat can therefore flow through these channels. The maximum thermal conductivity was up to 70% higher than for unfilled PEc at a talc concentration of 35 wt %. The talc particles were well dispersed throughout the matrix, and the particles could not form a percolated conductive path. So these results show that the heat transfer occurred according to the dispersion mechanism, with no percolation.

Other studies show that the thermal conductivity of the particle used as filler is not always as relevant. If we compare the addition of copper particles it has been shown that although copper (Cu) has a thermal conductivity that is about 20 times higher than for talc, the thermal conductivity of the a polymer composition comprising talc have higher thermal conductivity than the corresponding fraction of copper particles. The interconnectivity of the filler and matrix is thereby of large importance for the thermal conductivity of the compounded polymer composition.

FIG. 9 is a graph showing volumetric and specific heat capacity (Cp). What can be seen in FIG. 9 is that both volumetric and specific heat capacity decreased with the talc increment. From the application point of view, this means that improving heat transfer in the melt by introducing talc particles leads to a faster production rate, which would be important in terms of production output and cycle time.

FIG. 10 a is a graph showing thermal resistance in a geothermal pipe collector as a function of the thermal conductivity of the polymer composition of which the pipe collector is manufactured. The thermal resistance of the pipe collector is dependent on the physical properties of the pipe, such as diameter, wall thickness and the occurrence of patterns increasing the heat exchange.

FIG. 10 b shows the thermal resistance as a function of thermal conductivity for four pipe collectors having different diameter and wall thickness. From this graph, it is clear that the thermal resistance of the geothermal pipe collector decreases as the wall of the geothermal pipe collector is made thinner (as also shown in FIG. 10 a) and the thermal conductivity of the material increases. The result of which is that by increasing the mechanical properties of the polymer composition and increasing the thermal conductivity of the pipe, the thermal resistance of the pipe collector could be substantially reduced, which makes it possible to get the same heat exchange from the ground using a shorter pipe collector.

FIG. 11 is a table showing the result of Thermogravimetric analysis showing the CB is an effective additive for increasing thermal stability. A decomposition of PEn of 5% occurred at 395° C. while 5% degradation in weight for PEc occurred at 435° C. Additionally, the maximum mass loss temperatures were 448° C. and 462° C., respectively. One explanation for the higher thermal stability for PEc might be the moderate enhancement of the thermal conductivity and the uniform heat dissipation. Apart from that, CB has a non-polar surface character, which is more compatible in a matrix like HDPE, as it is also non-polar. Thus, the interfacial heat transfer could be improved, reducing local overheating and hot spots, which can delay the thermal degradation. The table of FIG. 11 also shows that almost 100% of the polyethylene in all samples degraded at 550° C., and that the residue contained CB and talc. At 600° C., the CB became oxidized, which left the talc as a residue. The amount of CB can therefore be calculated by subtracting the weight loss at 650° C. from that at 3 600° C. The amount of final residue increased correspondingly with the proportion of filler. The calculated percentage of residue for CB and talc for each composite was in accordance with the values given in the table of FIG. 2.

FIG. 12 shows a graph of thermal degradation (weight as a function of temperature) for PEn, PEc and PE with different amounts of talc. As can be seen, PEn started to degrade at a temperature of 395° C., and decomposition of almost 100% occurred at 570° C. As can be seen from the graph, at high loadings, the composition comprising talc has lower thermal stability than the PEc. The mechanism of accelerated degradation can be explained in two ways. Firstly, the ability of the talc particle surface to absorb stabilizers can result in reduced long-term thermal stability. Therefore, as the specific surface area of the filler is increased, this adverse effect can be more pronounced.

Apart from increasing the thermal stability, CB increases light stability and protection against UV. However, it has been shown that the UV-degradation of polymer compositions comprising talc is accelerated, meaning that compositions with high loadings of talc are more sensitive the natural environment. Therefore, when choosing the talc loaded polymer composition for a geothermal pipe collector, the increase in thermal conductivity must be weighed against the problems with accelerated degradation of the material.

FIG. 13 shows a graph of the particle size distributions with respect to the cumulative volume and the volume in each size fraction (particle diameter as a function of volume). The mean particle size was 11.14+/−0.02 μm.

FIG. 14 a shows a geothermal pipe collector 12 according to one embodiment in which the inner surface 14 of the pipe collector 12 comprises recesses or protrusions 16 extending helically in relation to the length axis (L in FIG. 14 b) of the pipe collector 12, such that a turbulent flow is created in the direction of the length axis of the pipe collector. The recesses or protrusions 16, may be continuous or discontinuous in the longitudinal direction of the single pipe collector 12. Usual dimensions for geothermal pipe collectors 12 are within the range 25-63 mm in diameter. The height of the indentations and/or elevations 16, which could be grooves 18 or the grooving, can be varied, but can typically be within the range of 0.2-5 mm depending on the size of the pipes and the wall thickness, and preferably within the range 0.2-2 20 mm, for the most usual dimensions of the collector pipes 12. The grooves 18 are evenly spread around the inner circumferential surface of the pipe, as seen in the cross section of FIG. 14 a. The creations of grooves on the inner surface of the pipe collector makes the wall of the pipe collector thinner at some portions, which may affect some of the mechanical properties of the pipe collector 12. Thus, when removing material from the wall of the pipe collector, it may be necessary or advantageous to increase the mechanical properties of the material of which the pipe collector is made.

14 b shows one embodiment of the geothermal pipe collector 2 in a longitudinal section. The geothermal pipe collector comprises helically extending recesses or protrusions 16 which alter direction at least at some portion along the length axis L of the pipe collector 2. The direction of the helical shape of the recesses or protrusions 16 can be altered suitably at least every second meter or every meter, in the longitudinal direction L of the pipe.

Examples of how a turbulent flow can increase the heat exchange in a geothermal pipe collector can be found in for example EP 2195586 to M. Ojala et al.

Please note that any of the polymer compositions or any combinations of additives mentioned herein could be used with any type of geothermal pipe collector without departing from the basic idea of the invention. 

1. A geothermal pipe collector made from a polymer composition comprising: more than 50 wt % polyethylene, 0.1 wt %-35 wt % talc and 0.5 wt %-10 wt % carbon black.
 2. The geothermal pipe collector according to claim 1, wherein the polymer composition comprises 0.1 wt %-3 wt % talc.
 3. The geothermal pipe collector according to claim 1, wherein the polymer composition comprises 8 wt %-35 wt % talc.
 4. The geothermal pipe collector according to claim 1, wherein the polymer composition comprises 8 wt %-15 wt % talc.
 5. The geothermal pipe collector according to claim 1, wherein the polymer composition comprises 8 wt %-12 wt % talc.
 6. The geothermal pipe collector according to claim 1, wherein the polymer composition comprises 0.5 wt %-5 wt % carbon black.
 7. The geothermal pipe collector according to claim 1, wherein the polymer composition comprises 1.5 wt %-3 wt % carbon black.
 8. The geothermal pipe collector according to claim 1, wherein the polymer composition comprises more than 75% polyethylene.
 9. The geothermal pipe collector according to claim 1, wherein the talc is talc in which the average aspect ratio is above 1.2.
 10. The geothermal pipe collector according to claim 1, wherein the inner surface of the pipe collector comprises recesses or protrusions for increasing the turbulence of a medium flowing in the pipe collector.
 11. The geothermal pipe collector according to claim 10, wherein the recesses or protrusions extends helically on the inner surface of the pipe collector, in relation to the length axis of the pipe collector, such that a turbulent flow is created in the direction of the length axis of the pipe collector.
 12. The geothermal pipe collector according to claim 11, wherein the helically extending recesses or protrusions alter direction at least at some portion along the length axis of the pipe collector.
 13. The geothermal pipe collector according to claim 10, wherein the recesses or protrusions extend continuously on the inner surface of the pipe collector.
 14. A method, comprising: using the polymer composition, defined in claim 1, for increasing thermal conductivity of a geothermal pipe collector.
 15. A method, comprising: using the polymer composition, defined in claim 1, for increasing mechanical properties of a geothermal pipe collector. 